1. Field of the Invention
The present invention relates to a hydrodynamic pressure bearing system for a spindle motor or the like for driving a magnetic disk of a hard disk drive of a computer, and a spindle motor using the same.
2. Description of the Related Art
Recently, in hard disk drives of computers, bearings of spindle motors for driving magnetic disks are required to have high rotation accuracy, low friction, low noise, and long life as the magnetic disks have higher density and higher rotation speed. In order to achieve such requirements, spindle motor using a hydrodynamic pressure bearing system are developed.
The hydrodynamic pressure bearing system seals fluid between a shaft and a sleeve for supporting the shaft, and forms dynamic pressure grooves at the caliber of the sleeve, thereby generating dynamic pressure in the fluid by the rotation of the shaft and supporting the shaft in a floating manner by the dynamic pressure. The hydrodynamic pressure bearing system has a fluid layer formed between the shaft and the sleeve, and can thus support the shaft without mechanical friction by keeping them from contact with each other. Therefore, high rotation accuracy, low friction, low noise, and long life can be achieved.
However, the conventional hydrodynamic pressure bearing systems have the following problems: In general, the shafts and the sleeves of hydrodynamic pressure bearing systems of spindle motor for hard disk drives are made of stainless steel, having a high thermal coefficient of expansion, thus causing change in dimension with temperature to vary the clearance therebetween. The variations in the clearance between the shaft and the sleeve exerts a direct influence on the dynamic pressure of the fluid, causing a decrease in rotation accuracy and also the leakage of the fluid sealed in the clearance. In this case, since the shaft or sleeve on the coil side of the spindle motor easily increases in temperature, causing a temperature gradient between the shaft and the sleeve and increasing dimensional differentials due to thermal expansion, thus posing a serious problem.
Since the clearance at the rotating portion of the hydrodynamic pressure bearing system is extremely small, the existence of fine foreign matter generates friction to cause defects such as abnormal rotation and locking. Also, when there is roughness on the surfaces of portions forming the clearance, foreign matter such as fine burrs formed during processing may drop into the clearance during the use of the bearing system. Therefore, it is necessary to polish the surfaces of the portions forming the clearance to a mirror-smooth state after cutting, incurring high cost.